1. Technical Field
The present invention relates to a power semiconductor device or module applied to a multi-level power conversion system of three levels or more, and to a power conversion system to which the module is applied.
2. Related Art
FIG. 11 shows an example of a circuit of a three level inverter, which is a power conversion circuit that converts from a direct current to an alternating current. In a direct current power source in which C1 and C2 are connected in series (a large capacity capacitor may be used instead), a positive side potential is Cp, a negative side potential is Cn, and intermediate point potentials are Cm (Cm1 and Cm2). Generally, when the direct current power source is configured from an alternating current power source system, it is possible to configure by applying a rectifier, a large capacity electrolytic capacitor, or the like.
Reference numerals 3 and 4 are an IGBT and diode of an upper arm connected to the positive side potential Cp, reference numerals 5 and 6 are an IGBT and diode of a lower arm connected to the negative side potential Cn, and the upper arm and lower arm are connected in series, configuring a one phase arm. A three phase circuit is configured of three phase arms. Also, reference numerals 7, 8, 9, and 10 are elements configuring a bidirectional switch connected between the direct current power supply intermediate point potential Cm (Cm1 and Cm2) and an alternating current output terminal 11, wherein 7 and 8 are IGBTs, and 9 and 10 are diodes. The bidirectional switch shown in FIG. 11 is of a configuration wherein IGBTs to which a diode is reverse parallel connected are connected in reverse series, and is applied to each phase. In the drawing, the IGBT 7 and IGBT 8 are connected in reverse series with a common emitter, but the switch can also be realized with a common collector configuration or, as shown in FIG. 13B, with a configuration wherein IGBTs 12 and 13 having reverse blocking voltage are reverse parallel connected.
Lo is a filter reactor, and 2 is a load of the system. By adopting this circuit configuration, it is possible to output the direct current power source positive side potential Cp, negative side potential Cn, and intermediate point potential Cm, to the output terminal 11. That is, the circuit is a three level inverter circuit that outputs three levels of voltage waveform. FIG. 12 shows an example of an output voltage (Vout) waveform. A characteristic being that there are less low order harmonic components (close to a sinusoidal waveform) than with a two level inverter, it is possible to miniaturize the output filter reactor Lo.
Also, FIG. 14 shows a double converter type of power conversion system configured of a PWM converter (CONV) that converts alternating current to direct current and a PWM inverter (INV) that converts direct current to alternating current. A configuration is such that, with a three phase alternating current power source 1 as an input, a stable alternating current voltage is generated by an input filter reactor Li, the three phase three level PWM converter CONV, large capacity capacitors C1 and C2 connected in series, the three phase three level PWM inverter INV, and an output filter Lo, and alternating current power is supplied to a load 2.
An example of a case of configuring the three level converter (converter or inverter) with a dedicated IGBT module is shown in Japanese Patent Publication No. JP-A-2008-193779. FIG. 15B shows an external structural view of the module, and FIG. 15A shows an example of an internal circuit. Reference numerals 24, 25, 26, and 27 are, respectively, a terminal P connected to the potential Cp, a terminal M connected to the potential Cm, a terminal N connected to the potential Cn, and an alternating current output terminal U. It is possible to configure a three phase inverter by using three of this module, and when seeking a still larger capacity, this can be realized by connecting the modules in parallel.
FIG. 16 shows an equivalent circuit described focusing on internal wire inductors (L1 to L5) of the module of FIGS. 15A and 15B. Each inductor is mainly formed by the wires between the module output terminal and semiconductor chips and between the semiconductor chips. As each wire is normally of around a few centimeters, each inductance value is around 10 nH.
FIG. 17 is a circuit diagram for illustrating the problem. In FIG. 17, when an IGBT T1 is in an on condition, a current I flows along the path (a path passing from a capacitor C1, through an inductor L1, the IGBT T1, and an inductor L3, to a reactor Lo) shown by the dotted line. Next, on the IGBT T1 being turned off, an IGBT T4 turned on in advance has continuity, and the current of the reactor Lo is transferred to a current path 28 passing from the reactor Lo, through an inductor L2 and the IGBT T4, to the reactor Lo. At this time, a voltage is transiently generated in the directions of the arrows in the drawing in the inductors L1, L2, and L3, in accordance with an IGBT current change rate (di/dt).
As a result, when ignoring a wire inductance of external wires, a maximum of the voltage shown in Equation 1 (shown below) is applied between the collector and emitter of the IGBT T1. FIG. 18 shows waveforms of a collector current (ic) and a voltage between the collector and emitter (VCE) when the IGBT T1 is turned off.VCE(peak)=Edp+(L1+L2+L3)·di/dt  Equation 1Surge voltage ΔV=(L1+L2+L3)·di/dt  Equation 2                Where:        Edp: direct current power source 1 direct current voltage        di/dt: IGBT current change rate when IGBT is turned off        L1+L2+L3: wire inductance value        
As one example, in the case of an IGBT in the 100 ampere class, as the di/dt thereof is a maximum of around 2,000 A/μs, when L1+L2+L3=30 nH, the surge amount (L1+L2+L3)·di/dt according to Equation 1 is 60V.
Consequently, due to the existence of L1, L2, L3, L4, and L5, the value of the peak voltage applied to the IGBT when the IGBT is turned off increases with respect to the direct current voltage Edp by the amount of the surge voltage in Equation 2, meaning that the IGBT chip and the chip connected in parallel thereto need to be ones with a high voltage rating. Normally, a chip with a high voltage rating is such that the chip area increases roughly in proportion to the voltage rating, meaning that the module increases in size, and the cost increases.
In particular, when seeking an increase in module current (an increase in capacity), the volume of the module increases, meaning that the length of wiring in the module inevitably increases, as a result of which the wire inductance value also increases. Also, as the di/dt when switching also increases in approximate proportion to the current value, the surge voltage ΔV according to Equation 2 increases exponentially with respect to an increase in the current rating of the module. For this reason, a limit arises in the achieving of an increase in capacity in one module. Meanwhile, although an increasing of capacity is routinely carried out by connecting modules in parallel, it is necessary to consider the increase in cost compared with the case of configuring with one module, and the imbalance in current between the parallel connections, meaning that there is a problem in that the parallel connection has to be derated during design.